The present invention relates to a plasma display device employing a plasma display panel (hereinafter referred to as a PDP) and a method of driving the PDP. The present invention is useful for improving luminous efficacy of the PDP and suppressing deterioration of protective films within the PDP with operating time of the PDP.
Now, as plasma TV (PDP-TV) receivers, which are one kind of plasma display devices employing a plasma display panel (PDP), are establishing themselves in the market of thin, large-screen TV receivers, they are competing fiercely against competitive devices such as liquid crystal display devices and others.
FIG. 10 is an exploded perspective view of an example of a conventional ac surface-discharge type PDP employing a three-electrode structure. In the ac surface-discharge type PDP shown in FIG. 10, a discharge space 63 is formed between a pair of opposing glass substrates, a front substrate 51 and a rear substrate 58. Usually the discharge space 63 is filled with a discharge gas at several hundreds or more of Torr. As the discharge gas, usually He, Ne, Xe, and Ar are used either alone or in combination with one or more of the others.
A plurality of sustain electrode pairs of X and Y electrodes which generate discharge mainly for display light emission are disposed on the underside of the front substrate 51 serving as a display screen.
Usually, each of the X and Y electrodes is made of a combination of a transparent electrode and an opaque electrode for supplementing conductivity of the transparent electrode.
The X electrodes 64 are comprised of transparent X electrodes 52-1, 52-2, . . . and corresponding opaque X bus electrodes 54-1, 54-2, . . . , respectively, and the Y electrodes 65 are comprised of transparent Y electrodes 53-1, 53-2, . . . and corresponding opaque Y bus electrodes 55-1, 55-2, . . . , respectively. It is often that the X electrodes are used as a common electrode and the Y electrodes are used as independent electrodes. Usually a discharge gap (also called a slit or a regular slit) Ldg between the X and Y electrodes in one discharge cell is designed to be small such that a discharge start voltage is not excessively high, and a spacing (also called a reverse slit) Lng between an X electrode in one cell and a Y electrode in another cell adjacent to the one cell is designed to be large such that unwanted discharge is prevented from occurring between two adjacent cells.
The X and Y sustain electrodes 64, 65 are covered with a front dielectric substance 56, a surface of which, in turn, is covered with a protective film 57 made of material such as magnesium oxide (MgO) or the like.
The MgO protects the front dielectric substance 56 and lowers a firing voltage because of its higher sputtering resistance and higher secondary electron emission yield, compared with other materials.
Address electrodes (also called A electrodes) 59 for addressing cells and thereby generating address-discharge are arranged on the upper surface of the rear substrate 58 in a direction perpendicular to the sustain electrodes (X and Y electrodes). The A electrodes 59 are covered with a rear dielectric substance 60. Ribs 61 are disposed between adjacent A electrodes 59 on the rear dielectric substance 60. A phosphor 62 is coated in a cavity formed by the wall surfaces of the ribs 61 and the upper surface of the rear dielectric substance 60.
In this configuration, each of intersections of the sustain electrode pairs with the A electrodes corresponds to one discharge cell, and the discharge cells are arranged in a two-dimensional fashion. In a color PDP, a trio comprised of three kinds of discharge cells coated with red, green and blue phosphors, respectively, forms one pixel.
FIG. 11 and FIG. 12 are cross-sectional views of one discharge cell shown in FIG. 10 viewed in the directions of the arrows D1 and D2, respectively. In FIG. 12, the boundary of the cell is approximately indicated by broken lines. In FIG. 12, reference numeral 66 denote electrons, 67 is a positive ion, 68 is a positive wall charge, and 69 are negative wall charges.
Next operation of the PDP of this example will be explained.
The principle of generation of light by the PDP is such that discharge is started by a voltage pulse applied between the X and Y electrodes, and then ultraviolet rays generated by excited discharge gases are converted into visible light by the phosphor.
FIG. 13 is a block diagram illustrating a basic configuration of a plasma display device 100. The PDP (also called the plasma display panel or the panel) 91 is incorporated into the plasma display device 100. The PDP 91 is coupled to a driving circuit 98 which is comprised of an X driving circuit 95, a Y driving circuit 96 and an A driving circuit 97 for supplying required voltages to the X, Y and A electrodes, respectively, via an X electrode terminal portion 92, a Y electrode terminal portion 93 and an A electrode terminal portion 94 which serve as connecting portions between electrode groups within the panel and external circuits.
The driving circuit 98 receives signals for a display image from a video signal source 99, converts the signals into driving voltages, and then supplies them to respective electrodes of the PDP 91. Illustrated in FIGS. 14(a)-14(c) are concrete examples of the driving voltages in a case where the ADS (Address Display-Period Separation) scheme is employed for producing gray scale levels.
FIG. 14(a) is a time chart illustrating a driving voltage during one TV field required for displaying one picture on the PDP shown in FIG. 10. FIG. 14(b) illustrates waveforms of voltages applied to the A electrode 59, the X electrode 64 and the Y electrode 65 during the address period 80 shown in FIG. 14(a). The X electrode and the Y electrodes are called the sustain electrodes, and a pair of an X electrode and a Y electrode is called a sustain electrode pair. FIG. 14(c) illustrates sustain pulse voltages (also called sustain voltages or sustain pulses) applied to the X and Y electrodes, which are the sustain electrodes, all at the same time, and a voltage (an address voltage) applied to the address electrodes all at the same time, during the sustain period 81 shown in FIG. 14(a).
Portion I of FIG. 11(a) illustrates that one TV field 70 is divided into sub-fields 71 to 78 having different plural numbers of light emission from one another. Gray scales are generated by a combination of one or more selected from among the plural sub-fields.
Suppose the eight sub-fields are provided which have different gray scale brightness steps in binary number step increments, then each discharge cell of a three-primary color display device provides 28 (=256) gray scales, and as a result the three-primary color display device is capable of displaying about 16.78 millions of different colors.
Portion II of FIG. 14(a) illustrates that each sub-field comprises a reset period 79 for resetting the discharge cells to an initial state, an address period 80 for addressing discharge cells to be lighted, and a sustain period 81 for causing the addressed discharge cells to generate light.
FIG. 14(b) illustrates voltage waveforms (sustain pulse voltage waveforms) applied to the A electrode 59, the X electrode 64 and the Y electrode 65 during the address period 80 shown in FIG. 14(a). A waveform 82 represents a waveform (an A waveform) of a voltage V0 V applied to one of the A electrodes 59 during the address period 80, a waveform 83 represents a waveform (an X waveform) of a voltage V1 V applied to the X electrode 64, and waveforms 84 and 85 represent waveforms (Y waveforms) of voltages V21 V and V22 V applied to ith and (i+1)st ones of the Y electrodes 65, respectively.
As shown in FIG. 14(b), when a scan pulse 86 is applied to the ith row of the Y electrodes 65, in a cell located at an intersection of the ith row of the Y electrodes 65 with the A electrode 59 supplied with the voltage V0, first an address discharge occurs between the Y electrode and the A electrode, and then an address discharge occurs between the ith row of the Y electrodes 65 and the X electrode. No address discharges occur at cells located at intersections of the ith row of the Y electrodes 65 and with the A electrode 59 at ground potential.
The above applies to a case where a scan pulse 87 is applied to the (i+1)st one of the Y electrodes 65.
As shown in FIG. 12, in the cell where the address discharge has occurred, charges (wall discharges) are generated by the discharges on the surface of the dielectric substance 56 and the protective film 57 covering the X and Y electrodes, and consequently, a wall voltage Vw V occurs between the X and Y electrodes. As explained already, in FIG. 12, reference numeral 66 denote electrons, 67 is a positive ion, 68 is a positive wall charge, and 69 are negative wall charges. Occurrence of sustain discharge during the succeeding sustain period 81 depends upon the presence of this wall charge.
FIG. 14(c) illustrates sustain pulse voltages applied to the X and Y electrodes serving as the sustain electrodes all at the same time during the sustain period 81 shown in FIG. 14(a). The X electrode is supplied with a sustain pulse voltage of a voltage waveform 88, the Y electrode is supplied with a sustain pulse voltage of a voltage waveform 89, and the magnitude of the voltages of the waveforms 88 and 89 is V3 V The A electrode 59 is supplied with a driving voltage of a voltage waveform 90 which is kept at a fixed voltage V4 V during the sustain period. The voltage V4 may be selected to be ground potential. The sustain pulse voltages of the magnitude V3 is applied alternately to the X electrode and the Y electrode, and as a result the reversal of the polarity of the voltage between the X and Y electrodes is repeated. The magnitude V3 is selected such that the presence and absence of the wall voltage generated by the address discharge correspond to the presence and absence of the sustain discharge, respectively.
In a discharge cell where the address discharge has occurred, discharge is started by the first sustain voltage pulse, the discharge continues approximately until wall charges of the opposite polarity accumulate to cancel the applied voltage. Since the wall voltage accumulated due to this discharge has the same polarity as that of the second sustain voltage pulse of the polarity opposite from that of the first sustain voltage pulse, another discharge occurs again. The above is repeated after application of the third, fourth and succeeding sustain voltage pulses.
In this way, in the discharge cell where the address discharge has occurred, sustain discharges occur between the X and Y electrodes the number of times equal to the number of the applied voltage pulses and thereby they emit light. On the other hand, light is not generated in the discharge cells where the address discharge has not occurred.
The above is the basic configuration of the conventional plasma display device and its conventional driving method.
With the advent of competitive devices in the market for thin large-screen TV receivers, the improvement of luminous efficacy of the PDP is becoming increasingly important. As reported in “High Efficacy PDP,” SID 03, pp. 28-31, increasing of the partial pressure of Xe in the discharge gas of the PDP is known as a means for improving the luminous efficacy of the PDP. However, since a driving voltage (a sustain voltage) is increased by increasing of the partial pressure of Xe in this method, there arises a problem in that the amount of ion bombardment induced sputtering from the protective film is increased, and consequently, the lifetime is decreased. In general, as measures against the increase in the amount of ion bombardment induced sputtering due to an increase in sustain voltages, reported are methods of improving the protective films such as a method by increasing the thickness of the protective film, and a method by using the protective film having a high secondary electron emission coefficient. By way of example, JP 2003-151446 A discloses a method of lowering driving voltages by using a two-layer protective film of CaO/MgO and lengthening a lifetime of the protective film by increasing its thickness, and JP 2004-71367 A discloses a method of lengthening a lifetime of the protective film by lowering driving voltages by fabricating the protective film from a material (diamond) other than MgO. However, it is thought that there are various problems with putting those protective films to practical use. Therefore there have been demands for a method of suppressing deterioration of protective films over operating time of the PDP, other than the method of improving the protective films.